Crystallinity isomorphism

In the present paper, we show that the quantum chemical (QC) simulation can be used as a simple and efficient tool for testing, whether known or theoretically predicted solid tends to form an amorphous phase. Two well-known substances, representing the extreme examples of crystalline and amorphous solids, namely lithium fluoride (LiF) and silica (Si02), are considered in order to illustrate the methodology. LiF is crystalline and hardly forms glassy state, while Si02 is known to form a number of non-crystalline isomorphes [4,5]. 

In general, the chemistry of inorganic lead compounds is similar to that of the alkaline-earth elements. Thus the carbonate, nitrate, and sulfate of lead are isomorphous with the corresponding compounds of calcium, barium, and strontium. In addition, many inorganic lead compounds possess two or more crystalline forms having different properties. For example, the oxides and the sulfide of bivalent lead are frequendy colored as a result of their state of crystallisation. Pure, tetragonal a-PbO is red pure, orthorhombic P PbO is yeUow and crystals of lead sulfide, PbS, have a black, metallic luster. 

There are a considerable number of stable crystalline salts of the ammonium ion [14798-03-9] NH. Several are of commercial importance because of large scale consumption in fertiliser and industrial markets. The ammonium ion is about the same size as the potassium and mbidium ions, so these salts are often isomorphous and have similar solubiUty in water. Compounds in which the ammonium ion is combined with a large, uninegative anion are usually the most stable. Ammonium salts containing a small, highly charged anion generally dissociate easily into ammonia (qv) and the free acid (1). At about 300°C most simple ammonium salts volatilize with dissociation, for example... 

Cloud Seeding. In 1947, it was demonstrated that silver iodide could initiate ice crystal formation because, in the [ -crystalline form, it is isomorphic with ice crystals. As a result, cloud seeding with silver iodide has been used in weather modifications attempts such as increases and decreases in precipitation (rain or snow) and the dissipation of fog. Optimum conditions for cloud seeding are present when precipitation is possible but the nuclei for the crystalliza tion of water are lacking. 

Commerical grades of EVOH typically have vinyl alcohol contents in the range 56-71%, but in contrast to the corresponding EVA materials these copolymers are crystalline. Furthermore, an increase in the vinyl alcohol content results in an increase in such properties as crystalline melting point, tensile strength and tensile modulus together with a decrease in oxygen permeability. This is a reflection of the fact that the ethylene and vinyl alcohol units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1). 

Copolymerisation. Except in those rare cases where monomer segments are isomorphous (see Figure 4.11), copolymerisation, as usual, leads to less crystalline and frequently amorphous materials. As might be expected, these materials are tough, leather-like, flexible and, when unfilled, reasonably transparent. 

Tellurium-chalcogen-nitrogen chemistry is also burgeoning. Typical examples include the red crystalline Te(NSO)2, isomorphous with... 

Just as an example, the X-ray diffraction patterns of compression moulded samples of PVDF, poly(vinylfluoride), and of some VDF-VF copolymers of different compositions are shown in Fig. 17 [90]. The degrees of crystallinity of the copolymer samples (40-50%) are high and analogous to those of the homopolymer samples. This indicates a nearly perfect isomorphism between the VF and VDF monomeric units [90, 96], The diffraction patterns and the crystal structures of the copolymers are similar to those of PVF, which are in turn similar to the X-ray pattern and crystalline structure of the P form of PVDF. On the contrary, the X-ray pattern of a PVDF sample crystallized under the same conditions (Fig. 17 a) is completely different, that is typical of the non-piezoelectric a form [90]. 

Additional observations reflect on the high stability of the DTU host lattice and illustrate its versatile capability to form clathrates with different guests. The crystalline 1 1 complexes of DTU with diethylether (Fig. 29) and diethylamine are isomorphous with the structure of the propanamide adduct (Fig. 24). However, in these structures the hydrogen bonds do not form a continuous pattern but are, rather, confined to... 

In the four isomorphous structures the different guest components are located in a similar crystalline environment. Since the species involved are of nearly identical... 

At room temperature, trans-8 is formed, whereas at -78 °C mixtures of cis-8 and trans-8 are obtained. Pure cis-8, which rearranges to trans-8 at 20°C, is the product when Si2Cl6 is the starting material. Crystalline trans-8 is isomorphous with the respective Ge- and Sn- complexes (d Si-Cl 2.27, d Si-P 2.35 A). 

The oxidation by electrocrystallization of the d2 anionic complexes [Cp M (dmit)2] with M = Mo or W afforded in both cases the neutral radical species [Cp M(dmit)2] on the electrode [42, 43] as a crystalline material. The two compounds are isomorphous. As shown in Fig. 17, the absence of any counter ion... 

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